While Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, single-site (metallocene and non-metallocene) catalysts represent the industry's future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of α-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Single-site olefin polymerization catalysts having “open architecture” are generally known. Examples include the so-called “constrained geometry” catalysts developed by scientists at Dow Chemical Company (see, e.g., U.S. Pat. No. 5,064,802), which have been used to produce a variety of polyolefins. “Open architecture” catalysts differ structurally from ordinary bridged metallocenes, which have a bridged pair of pi-electron donors. In open architecture catalysts, only one group of the bridged ligand donates pi electrons to the metal; the other group is sigma bonded to the metal. An advantage of this type of bridging is thought to be a more open or exposed locus for olefin complexation and chain propagation when the complex becomes catalytically active. Simple examples of complexes with open architecture are tert-butylamido(cyclopentadienyl)dimethylsilylzirconium dichloride and methylamido(cyclopentadienyl)-1,2-ethanediyltitanium dimethyl: 
While improvements in long-chain branching and comonomer incorporation have been achieved versus Ziegler-Natta processes, there is a need for further improvements. With the currently available systems, the amount of long-chain branching is small and addition of comonomer adds to the cost.
Dehydrogenation catalysts form the basis for such processes as the manufacture of styrene from ethylbenzene. See March, Advanced Organic Chemistry 3rd ed. (1985) Wiley, NY, p. 1053 for a brief discussion of dehydrogenation. Commonly, dehydrogenation catalysts are similar to or the same as hydrogenation catalysts since the dehydrogenation reaction is the reverse of hydrogenation. Platinum, palladium, and nickel are common, and the reactions proceed only at very high temperatures (in excess of 300° C.) since the dehydrogenation is highly endothermic. The dehydrogenation of ethylbenzene is performed at about 620° C. over catalysts which primarily consist of iron oxide and potassium salt promoters. For some other typical examples of high-temperature catalytic dehydrogenation, see U.S. Pat. No. 3,903,191 (uses greater than 800° F.) or Catal. Today 51 (1999) 223 (chromium oxide on alumina; >580° C.).
During the 1980s, Professor Robert Crabtree and others discovered that certain iridium complexes are capable of catalytically dehydrogenating alkanes to alkenes under exceptionally mild thermal (i.e., less than 160° C.) or even photolytic conditions (see, e.g., J. Am. Chem. Soc. 104 (1982) 107; 109 (1987) 8025; J. Chem. Soc., Chem. Commun. (1985) 1829). For a more recent example, see Organometallics 15 (1996) 1532.
In addition, “pincer” complexes of platinum-group metals have been known since the late 1970s (see, e.g., J. Chem. Soc., Dalton Trans. (1976) 1020). Pincer complexes have a metal center and a pincer skeleton. The pincer skeleton is a tridentate ligand that is connected to the metal via at least one metal-carbon sigma bond; substituents ortho to this sigma bond are held in a fixed position and can coordinate to the metal site. The use of pincer complexes in organic synthesis, including their use as low-temperature alkane dehydrogenation catalysts, was exploited during the 1990s and is the subject of two excellent review articles (see Angew. Chem. Int. Ed. 40 (2001) 3751 and Tetrahedron 59 (2003). See also U.S. Pat. No. 5,780,701. Jensen et al. (Chem. Commun. 1997 461) used iridium pincer complexes to dehydrogenate ethylbenzene to styrene at 150 to 200° C. Recently, pincer complexes have been developed that dehydrogenate hydrocarbons at even lower temperatures. For some recent examples, see J. Mol. Catal. A 189 (2002) 95, 111 and Chem. Commun. (1999) 2443.
Polymerization catalysts based upon Group 4–9 first or second row transition metals having tridentate ligands are disclosed in U.S. Pat. No. 6,294,495. The tridentate ligands can be pincer ligands, but the use of dehydrogenation catalysts is not disclosed. The only complexes disclosed are polymerization catalysts.
Despite the extensive research into new methods to prepare polyolefins, no one has apparently contemplated polymerizing olefins in the presence of both an olefin polymerization catalyst and a low-temperature dehydrogenation catalyst. On the other hand, in-situ dehydrogenation would generate alkenes readily available for incorporation into a growing polyolefin chain.